This invention relates generally to yarn winding and more particularly, to winding cylindrical yarn packages with improved formation and stability. Such packages are commonly formed by windups employing a surface drive using the "print roll" or "direct" traversing techniques. A distinction should be made between "print roll" winding, and "direct" traversing. Both are commonly found in the art, but there are differences both in the application of the reversal profiles to the package and in the resulting package structure that are important to an understanding of this invention.
In "print roll" winding, a controlling traverse means generates a curved yarn reversal on the featureless cylindrical surface of the "print roll" that passively carries the yarn to the package and rolls it onto the package surface. Because only friction with the roll surface maintains the curved yarn reversal on the roll, usually the radius of the reversal must be relatively large and the helix angle of the yarn between curved yarn reversals relatively low. The large reversal radius tends to generate high shoulders on the ends of the package, which is reduced by "ironing" contact with the print roll. Because the reversal radius is large and the helix angle is low, little axial component of tension exists, and the ironing results in the excess shoulder yarn being displaced outward, forming bulged ends and increasing the package length beyond the stroke length of the reversal as carried by the print roll; therefore, with increasing package size, newly laid curved yarn reversals generally fall inside the outer limits of the package length and their stability on the package depends solely on friction with the substantially cylindrical surface of the package again requiring a relatively large reversal radius.
In the "direct" traverse method used in this invention and the invention of U.S. Pat. No. 4,136,836, the controlling element (in this case the inner shoulder of the roll groove) leads the yarn onto the package by means of tangential laydown from the roll reversal groove to the package; the yarn reversal curves so generated, that define the end of the package, called outer reversals, may be of small radius and may lead away from and into a relatively high helix angle, and, as a result, shoulder buildup and consequent ironing and package lengthening effects are minimized. Newly laid yarn is led slightly beyond the yarn reversals of the underlying layers with the result that the preponderance of these reversals tend to be "hooked" slightly over the end as may be seen by inspection of the package. This condition resembles an "overthrown end" except that the chordal length is so small as to be insignificant and harmless; however, the "hooking" contributes beneficially in that the newly laid reversal curves are to a degree mechanically constrained against sliding inward and in turn they constrain the package layers immediately below against being displaced outward by the ironing effect of the roll. In this invention, this effect is strengthened by the fact that outer reversals are of small radius and other reversals may be deposited inwardly, as will be described, substantially eliminating the shoulder that otherwise would be "ironed" outward, and additionally, the axial component of tension of the elastic yarn, acting on the "hooked" reversal, has a slight tendency to draw the layers of yarn below toward the center of the package, this insuring that subsequent reversals will continue to be "hooked." Reversals laid inboard are not so hooked and, therefore, in order to insure stability must be laid in a larger radius.
In any dual mode traverse windups employing a grooved or a partially grooved traversing roll as a second traversing device, the package may be driven or its surface speed may be controlled by direct surface contact with the grooved traversing roll which is operated at constant speed. As the yarn package increases in size its circumference increases, passing through a series of fractions of the yarn wave length (i.e., the length of one complete traverse cycle) as defined by the grooved traversing roll. During such condition as well as immediately prior to and following each such condition successive yarn waves on the package overlap or lie closely adjacent to each other producing a satiny, slippery surface known as a ribbon that is highly undesirable, particularly in the curved reversal portions where the yarn may become unstable and initiate sloughing toward the center of the package. The conditions under which the ribbons form are usually expressed as a ratio: ##EQU1## Typical numerical values being 3:1, 4:1, 5:2 and the like. When the terms of the ratio are low numbers the condition is more undesirable. Heretofore, in this type winding, the ribbon effects could not be avoided but only minimized by the expedient of running between package diameter limits corresponding to two consecutive low-ordered ribbons, e.g., 13:2 and 6:1; however, this imposes an intolerable limit on package size. Even in a dual mode winder in which the package being wound is not in contact with the grooved roll but is driven independently, ribbon formation could only be modified either by varying the speed of the traverse mechanism cyclically, as usually practiced, or by varying the surface speed of the package. These are feasible ways of partially avoiding formation of ribbons; however, added complexity and expense are incurred since some kind of speed changing devices and a means for programming the devices must be added. A further disadvantage is that due to rotational inertia it is difficult to effect a rapid change in speed. An alternative is to accept a slower speed change; however, it will be realized that the transitions from acceleration to deceleration and vice versa will inevitably be initiated while running in a ribbon wind condition, therefore, the speed change will not be made quickly enough to avoid ribbons completely or modify them significantly; thus, at times this type of ribbon breaking is ineffective for its intended purpose. This is particularly true as winding speeds are increased since the energy required increases as the square of the speed.